OF ASCERTAINING THE PRESENCE OF DIFFERENT METALS. 153 
crease of ^th part of a grain *. The platina was put into a small retort, 
which was then exhausted, twice filled with pure carbonic acid gas, and heated 
over a jar of the same gas, until all the arsenic rose and condensed in the upper 
part of the bulb of the retort as an extremely delicate whitish film, exhibiting 
no metallic lustre, even by the aid of a magnifying glass ; nor could this be 
expected from the minute quantity of arsenic present, and the extent of surface 
over which it was spread by sublimation. The platina was found perfectly 
clean, and of precisely the same weight as at first. Some pure water was put 
into the retort, and occasionally agitated in contact with the sublimate, but 
after thirty hours a recent solution of sulphuretted hydrogen occasioned no 
change in the water. The actual quantity of arsenic attached to the platina 
in the foregoing experiment, was ascertained to be y^th part of a grain. But 
this is very far from conveying a just idea of the degree to which this mode of 
detection may be carried : for a single drop of the aqueous arsenious acid 
would have afforded ample evidence of the arsenic as it respected colour, inso- 
lubility in muriatic and sulphuric acids, alliaceous odour, and volatility ; which 
would give the 2 to oth part of a grain. Even this very minute quantity gives us 
by no means the extreme limits to which this truly microscopic method of de- 
tecting metals may be carried. It is, however, quite unnecessary to pursue 
the subject further. 
In cases when the small platina crucible was used, the results were equally 
delicate, and much more simple and satisfactory. I put a single drop of 
aqueous arsenious acid, with about an equal bulk of muriatic acid, into the 
crucible ; the zinc being applied for an instant, the arsenic was reduced on 
the platina. The crucible was then rinsed with pure water, dried, covered 
with a piece of plate glass, and heated with a spirit lamp ; arsenious acid rose 
and condensed on the glass, whilst the surface of the crucible remained un- 
changed. The results were precisely similar, when a single drop of arsenic 
acid, of arsenite and of arseniate of potash, were exposed to similar treat- 
ment. 
I put T<joth part of a grain of solid arsenious acid into a very small platina 
crucible, which I coated with gold on the inside, and mixed with it about half 
* In my experiments I used a very delicate balance of Robinson’s construction, which turns with 
the rn'n vth part of a grain, when loaded with one hundred grains. 
MDCCCXXXI. 
X 
